Wifi phone not registering on network

[Desktop]

I live in an area with no O2 coverage although the status checker says there is. I use wifi phone to make calls. Recently I have been getting "No mobile network" when trying to call. I then have to drive 2 miles up the road to get the sim card to register on the O2 network. When I return home wifi phone works for awhile, then the mobile network registration is lost and I have to repeat the process. Is this an O2 issue, Android issue or Samgsung Galaxy A54 issue? Is there a fix?

[MI5]

@Desktop 

Have you set wifi preferred in the calling settings?

[Desktop]

Yes I have

[MI5]

@Desktop 

You could check the firmware is up to date on the phone and then reset your network settings. 

Forget your wifi connection on the phone, reboot your router and then reconnect.

If that fails to resolve, contact O2 for technical support, which might involve removing the wifi calling facility on your account and then adding it back again.

Message them on Social Media. Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG) or give them a call on 202 or 0344 809 0202

[Desktop]

I have made three calls to O2 Customer Services  getting nowhere. I have tied everything that's been suggested on here and by the Customer Services but I still have to drive 2 miles up to the top of the hill to enable Wifi Calling. There is no wifi or network coverage between the hill and my house yet when I get back wifi Calling is still enabled and I see the 5 bars greyed out instead of the circle with diagnal line. If I reboot, the network is lost and my Galaxy A54 won't register on the O2 network using Wifi Calling. In My O2 Device Details there is no option to turn on wifi calling and 4g. I read on here that the A54 firmware may be the issue. The CSC on my Galaxy A54 doesn't include ‘TEF’, ‘O2U’ or ‘OTF’. Customer Services insist nothing is restricted and wifi calling is enabled. 
Where can I find out how to change the firmwhere to be O2 compatible?

[MI5]

@Desktop 

O2 firmware doesn't exist anymore so not required.

It's just the crap way O2 have implemented wifi calling I guess.